In the clinical management of patients with an abdominal aortic aneurysm (AAA), physicians have long sought the ability to reliably identify patients with a high risk of rupture and those who are relatively safe from rupture. More precise information regarding rupture risk would allow early corrective surgery for some and avoid unnecessary surgery for others. The proposed project deals with a biomechanical approach to predicting rupture, using the fundamental mechanical principles used by engineers to assess the safety of a pressure vessel. The basic premise is that rupture occurs when the mechanical stress (i.e., internal forces per unit area) on the AAA wall exceeds the maximum stress that the aneurysmal tissue can inherently withstand (i.e., failure strength). Thus, if the wall stresses on a particular AAA is known, then one may make a reliable prediction regarding its rupture risk. Currently, the rupture risk is known to be related to AAA diameter and the patient's blood pressure, and both of these factors are important determinants of aneurysm wall stress. Additionally however, stress is also greatly dependent on aneurysm shape, which is seldom considered in a clinical setting. This is in part because the technology needed to precisely determine the AAA shape (and thus the wall stress) did not exist until recently. With new Computed Tomography (CT) imaging, computer-generated three dimensional reconstructions of CT data and mathematical techniques, AAA wall stress can now be calculated using a computer model. The proposed project will consist of three major elements. One element will test the basic hypothesis that wall stress is directly related to AAA rupture. Stress analysis will be performed on 20 AAAs near the time of rupture (in cases where CT data was coincidentally obtained within 1 month of rupture) and on 50 size-matched AAAs that did not rupture in a longer time span. Three-dimensional wall stress distribution (including peak, mean, and cyclic wall stresses) will be compared in the two groups. Another major element of the study will involve the analysis of wall stress distribution in at least 30 AAAs that have been followed with CT scans over extended periods (because they did not have indications for surgery). Wall stress distribution will be evaluated over time to examine the role of biomechanics in AAA expansion and progression. This analysis will include the role of three-dimensional shape and how it changes over time. The third element of the project will involve refinement of the noninvasive methodology used to determine AAA wall stress distribution. The CT data from the other two elements of the study will be re-analyzed to determine the utility of proposed refinements in computer modeling of human AAAs. The proposed project will likely be the first study to perform stress analyses on human AAA during progression and near the time of rupture. The study can be performed without harming patient care because it uses data that has already been obtained during normal clinical care (similar to existing clinical studies of AAA diameter). The results from this study will provide critical insight into the role of wall stresses during AAA disease progression and rupture, and may give clinicians a noninvasive tool to more